With the advance of the optical communication technology, a network using optical fibers has been rapidly built up. In the network, a wavelength multiplexing technique of collectively transmitting light beams having a plurality of different wavelengths has come into use, and a wavelength filter, a coupler, a waveguide, and the like have become important devices.
Some of the devices of the type described are changed in characteristics depending upon the temperature and may therefore cause troubles if used in the outdoors. This requires a technique for keeping the characteristics of these devices fixed or unchanged regardless of a temperature change, i.e., a so-called temperature compensation technique.
As a typical optical communication device which requires temperature compensation, there is a fiber Bragg grating (hereinbelow referred to as FBG). The FBG is a device in which a portion varied in refractive index in a grating-like pattern, i.e., a so-called grating is formed within a core of an optical fiber, and has a characteristic of reflecting a light beam having a specific wavelength according to the relationship represented by the following formula (1). Therefore, the device attracts attention as an important optical device in the optical communication system using a wavelength division multiplex transmission technique in which optical signals different in wavelength are multiplexed and transmitted through a single optical fiber.λ=2nΛ  (1)
Herein, λ represents a reflection wavelength, n, an effective refractive index of the core, and Λ, a grating period of the portion varied in refractive index in the grating-like pattern.
However, the above-mentioned FBG has a problem that the reflection wavelength will be varied following the change in ambient temperature. The temperature dependence of the reflection wavelength is represented by the following formula (2) which is obtained by differentiating the formula 1 with the temperature T.∂λ/∂T=2[(∂n/∂T)Λ+n(∂Λ/∂T)]=2Λ[(∂n/∂T)+n(∂Λ/∂T)/Λ]  (2)
The second term of the right side of the formula (2), i.e., (∂Λ/∂T)/Λ corresponds to a coefficient of thermal expansion of the optical fiber and has a value approximately equal to 0.6×10−6/° C. On the other hand, the first term of the right side corresponds to the temperature dependency of the refractive index of the core of the optical fiber and has a value approximately equal to 7.5×10−6/° C. Thus, it will be understood that the temperature dependency of the reflection wavelength depends upon both the variation in refractive index of the core and the change in grating period due to the thermal expansion but mostly results from the temperature-dependent variation of the refractive index.
As means for avoiding the above-mentioned variation in reflection wavelength, there is known a method in which the FBG is applied with tension depending upon the temperature change to thereby change the grating period so that a component resulting from the variation in refractive index is cancelled.
As a specific example of the above-mentioned method, proposal is made of a method in which the FBG is fixed to a temperature compensation member which comprises a combination of a material, such as an alloy or a silica glass, having a small coefficient of thermal expansion and a metal, such as aluminum, having a large coefficient of thermal expansion. Specifically, as illustrated in FIG. 1, an Invar (Registered Trademark) bar 10 having a small coefficient of thermal expansion has opposite ends provided with aluminum brackets 11a and 11b having a relatively large coefficient of thermal expansion attached thereto, respectively. An optical fiber 13 is fixed to the aluminum brackets 11a and 11b by the use of clasps 12a and 12b so that the optical fiber is stretched under a predetermined tension. At this time, adjustment is made so that a grating portion 13a of the optical fiber 13 is located between the two clasps 12a and 12b. 
If the ambient temperature rises in the above-mentioned state, the aluminum brackets 11a and 11b are expanded to reduce the distance between the two clasps 12a and 12b so that the tension applied to the grating portion 13a of the optical fiber 13 is decreased. On the other hand, as the ambient temperature falls, the aluminum brackets 11a and 11b are contracted to increase the distance between the two clasps 12a and 12b so that the tension applied to the grating portion 13a of the optical fiber 13 is increased. Thus, by changing the tension applied to the FBG depending upon the temperature change, it is possible to adjust the grating period of the grating portion. As a result, it is possible to cancel the temperature dependency of the reflection center wavelength.
However, the above-mentioned temperature compensation device is disadvantageous in that the structure is complicated and the handling is difficult.
As a method for solving the above-mentioned problems, WO97/28480 discloses a method of controlling the tension applied to an FBG 15 by fixing the FBG 15 to a glass ceramics substrate 14 obtained by heat treating and crystallizing a mother glass material preliminarily shaped into a plate and having a negative coefficient of thermal expansion, as illustrated in FIG. 2. In FIG. 2, the reference numeral 16 represents a grating portion, 17, an adhered and fixed portion, and 18, a weight.
Since the temperature compensation can be carried out by the single member, the method disclosed in WO97/28480 is simple in mechanism and easy to handle. However, the glass ceramics used therein is highly devitrifiable so that a resultant shape is restricted to a simple shape such as a plate. In other words, the member having a complicated shape can not be produced.
In addition, Japanese Unexamined Patent Publication JP 10-96827 A discloses a temperature compensation member made of a Zr-tungstate system material or a Hf-tungstate system material and having a negative coefficient of thermal expansion. However, since these materials are very expensive, it is difficult to put the disclosed one into practical use as an industrial product. Furthermore, in this temperature compensation member, the coefficient of thermal expansion is too large in a negative direction. This makes it difficult to successfully cancel the temperature dependency of the reflection center wavelength of the FBG. The coefficient of thermal expansion of the above-mentioned temperature compensation member can be adjusted in a positive direction by addition of a material, such as Al2O3, having a positive coefficient of thermal expansion. However, addition of the material such as Al2O3 decreases the strength as a result of a large difference in expansion among the materials used. It is therefore difficult to put the disclosed one into practical use as an industrial product.
It is therefore an object of this invention to provide a temperature compensation member which has a negative coefficient of thermal expansion, which can be shaped into even a complicated shape, and which can be manufactured at a low cost.
It is another object of this invention to provide an optical communication device using the above-mentioned temperature compensation member.